The purpose of this investigation is to ascertain if small undercut slots are as effective as threaded pins when used as retention in full crown build-ups. The slots are cut with 33 1/2 or 34 burs. Variations in slot depth and width are also being evaluated. Tensile tests have been used to compare the slot strength with that of pins. This mode has also been used to arrive at the best depth-width ratio. Initial results show that in a narrow slot, amalgam fractures at a higher p.s.i. value. In tensile tests the slots are slightly weaker than equivalent pins. In compression at 45 degrees to the tooth axis the slots are slightly stronger than the equivalent pins. A machine that simulates chewing has been made which measures the movement of a built-up crown on a tooth under varying loads and extended cycling. In this mode the slot-retained crowns required over twice the number of cycles to reach the same level deflection as did those with pins. Comparing amalgam with composite resin, the amalgam crowns withstood five times more cycles than the resin crowns to the same point.